1. Field of the Invention
This invention relates to a photosensitive sheet that comprises a light-reflective substrate in the shape of a sheet coated with pressure-rupturable capsules containing a chromogenic material and a photosensitive material, and to a method for the formation of images using the said photosensitive sheet.
2. Description of the Prior Art
Japanese Laid-Open Patent Publications 58-88739 and 59-149343 disclose methods for the formation of images that make use of photosensitive sheets, which are composed of a light-permeable substrate in the shape of a sheet and pressure-rupturable capsules coated on the substrate. The capsules contain a specific chromogenic material and a photosensitive material.
When this kind of photosensitive sheet is exposed to light from or through a certain image, only the pressure-rupturable capsules that were exposed on the top of the photosensitive sheet harden, resulting in a latent image on the photosensitive sheet that corresponds to the image from or through which the sheet was exposed to light. When the photosensitive sheet on which a latent image is formed is brought together with an image-receiving sheet that is coated with developing materials that give rise to colors with the chromogenic material inside the pressure-rupturable capsules, the pressure-rupturable capsules that are still not hardened rupture, and the chromogenic material contained in these pressure-rupturable capsules flows out. Then there is a reaction between the chromogenic material and the developing materials of the image-receiving sheet, which gives rise to color in the said chromogenic material, which forms an image on the image-receiving sheet.
It is also possible to form colored images by the use of the above-mentioned photosensitive sheet. For example, three kinds of photosensitive materials hardened by light are used that are hardened by wavelengths in the red, green, or blue regions, and one of the three kinds of photosensitive materials and a chromogenic material that gives rise to a color complementary to that color of light are contained in the same pressure-rupturable capsules. In this way, three kinds of pressure-rupturable capsules are prepared. That is, a material that is hardened by light with wavelengths in the blue region of light and a chromogenic material that gives rise to yellow, the color complementary to blue, are contained in some pressure-rupturable capsules (called Y capsules below); a material that is hardened by light with wavelengths in the green region of light and a chromogenic material that gives rise to magenta, the color complementary to green (called M capsules below); and a material that is hardened by light with wavelengths in the red region of light and a chromogenic material that gives rise to cyan, the color complementary to red (called C capsules below ) are the three kinds of pressure-rupturable capsules prepared. Then, these three kinds of pressure-rupturable capsules are distributed evenly over the top of the light-permeable substrate in the shape of a sheet to give a coated photosensitive sheet. This photosensitive sheet, when it is exposed to light reflected from a manuscript that is illuminated with white light, has the photosensitive material in each of the pressure-rupturable capsules of the said photosensitive sheet harden in response to the amount of light of the various wavelengths of the light for exposure, resulting in the formation on the photosensitive sheet of a latent image that has color information. Then, the photosensitive sheet that has this kind of a latent image is put together with pressure with an image-receiving sheet coated with developing materials that give rise to specific colors when they react with the chromogenic materials in the different pressure-rupturable capsules. In this way, the pressure-rupturable capsules on the said photosensitive sheet that have photosensitive material that is still not hardened are ruptured, and the chromogenic material in the inside flows out, reacting with the developing materials of the image-receiving material, giving rise to a specific color. The result is that a color image is formed on the image-receiving sheet that corresponds to the color image of the manuscript.
In this way, in the method for the formation of color images by use of a photosensitive sheet coated evenly with three kinds of pressure-rupturable capsules on top of a light-permeable substrate, it is not necessary to decompose the color images of the manuscript into the wavelengths of the light (blue, green, and red) to which the photosensitive materials of the different pressure-rupturable capsules are sensitive. Therefore, this method for the formation of images can be used to form a latent image that corresponds to the color image of the manuscript on the photosensitive sheet with exposure to light that is merely white light reflected from the manuscript.
However, the photosensitive material contained in the pressure-rupturable capsules of the photosensitive sheet is relatively insensitive to light, and for that reason, by simple exposure of the photosensitive sheet to the image of the manuscript, there is a possibility that the specific photosensitive material will not harden. Moreover, if the amount of light used for exposure is small, there is a possibility that the time taken for hardening of the photosensitive material will be long. In order to decrease these possibilities, the increasing of the amount of light used to expose the photosensitive sheet has been tried. However, to do this, it is necessary to increase the scale of the light source, to lengthen the time of exposure, and so on. Also, if the amount of light used for exposure of the photosensitive sheet is increased, photosensitive material that should not be hardened may become hardened.
The photosensitive materials that are contained in the pressure-rupturable capsules that coat the photosensitive sheet have individual light-sensitivities depending on their variety. For this reason, it is extremely difficult to have equal sensitivity to the different wavelengths of light when the three kinds of photosensitive materials are hardened by three different wavelengths of light. Therefore, the pressure-rupturable capsules that contain photosensitive materials with poor light-sensitivity are hardened with difficulty, and when these pressure-rupturable capsules are ruptured when the photosensitive sheet and the image-receiving sheet are placed together with pressure, there is the danger that the chromogenic material on their insides will give rise to color. In this case, it will not be possible to reproduce the color image of the manuscript faithfully.
In order to solve these kinds of problems, a method for the exposure of photosensitive sheets has been proposed in which white light is used to illuminate the manuscript, and the amount of light that reflects from the said manuscript is calibrated by filters. As shown in FIG. 10, the photosensitive sheet 10 that is used in this method is constituted of a light-permeable substrate 11 made of polyester film or the like and three kinds of pressure-rupturable capsules 12 that give rise to three different colors. The pressure-rupturable capsules 12 include C capsules that contain a chromogenic material that gives rise to the color cyan at the time of development, M capsules that contain a chromogenic material that gives rise to the color magenta at the time of development, and Y capsules that contain a chromogenic material that gives rise to the color yellow at the time of development. The different kinds of capsules are evenly distributed so as to coat the top of the substrate 11. This photosensitive sheet 10 is placed on the top of a sheet-exposure stand, and a manuscript 6 is illuminated with white light, so that the light that is reflected from the said manuscript 6 passes through a filter 3, exposing the sheet and selectively hardening the different pressure-rupturable capsules 12 (the pressure-rupturable capsules that are hatched in the figure are hardened). This forms a latent image with color information on the photosensitive sheet 10. The filter 3 functions to calibrate the sensitivity of the three kinds of pressure-rupturable capsules. Each kind of pressure-rupturable capsule has its own sensitivity characteristics, as shown, for example, in FIG. 11, wherein the sensitivity of the C capsules is greater than the sensitivities of the Y capsules and the M capsules. Therefore, even if the amount of red light to which the C capsules are sensitive is small, the said C capsules will harden. For that reason, in order to make the sensitivity of the three kinds of pressure-rupturable capsules the same, a filter that absorbs the red light to which the C capsules are sensitive is used, and the amount of said red light is decreased, which is the same in effect as increasing the amount of light in the wavelengths to which the Y capsules and the M capsules are sensitive. In this way, the apparent shift of the characteristic curve of the C capsules shown in FIG. 11 to the right can be attained. By this principle, if proper absorbance characteristics of red light by the filter 3 are selected, then, as shown in FIG. 9, it is possible to make the sensitivity characteristics of each of the kinds of pressure-rupturable capsules apparently approximately equal.
However, in this method for the formation of an image in which the sensitivity of the pressure-rupturable capsules are calibrated by the use of the above-mentioned filter, because light is absorbed by the filter, the overall amount of light that is used for the exposure to light of the photosensitive sheet is decreased. For this reason, it is necessary to increase the overall amount of white light used to illuminate the manuscript, and thus, it is necessary both to increase the size of the light source and to lengthen the time of exposure.